The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
In various forms of oil and gas production, produced or other water streams are created that must be treated for disposal or re-use. For example, in a bitumen mining process known as Steam Assisted Gravity Drainage (SAGD), steam is produced in a boiler or generator and injected into bitumen bearing soil. The steam reduces the viscosity of the bitumen allowing a mixture of water and bitumen to flow to a production well. After this mixture is extracted, most of the oil is removed in an initial primary oil-water separation step.
The remaining water, called produced water, is hot and alkaline, with a high pH. The produced water is treated through various unit operations to make it suitable for re-use in generating steam. The produced water contains, for example, residual oil, suspended and dissolved solids (organic and inorganic), and silica. The concentration of dissolved solids may be up to about 6,000 mg/L total dissolved solids (TDS) and the silica may be at or near the limit of solubility. Oil may be removed from the produced water by a second oil-water separation step such as gas flotation or a ceramic or walnut shell filter. Hot or warm lime softening may be used to remove silica and hardness. Particle filtration, for example through an anthracite bed, may be used to reduce total suspended solids. Strong or weak acid cation exchange softeners may be used to further reduce hardness.
After treatment, the produced water can be re-used to generate steam. The steam generators used in SAGD operations commonly include a Once Through Steam Generator that produces about 80% steam (vapour) and about 20% liquid droplets. The liquid fraction is removed from the steam in a blowdown stream before the steam is injected into the bitumen deposit. The OTSG blowdown water is further processed or, where permitted, disposed for example in a tailings pond or by deep well injection. One treatment option is to evaporate or vaporize generally pure water from the blowdown, for example in a brine concentrator followed by a crystallizer, to produce dried solids for disposal.